To　study　the　changing　of　characteristics　and　formation　mechanisms　of　PM2.5　in　Beijing　during　the　last　two　years,　particulate　matter　concentrations,　weather　conditions,　and　air-mass　trajectories　were　analyzed　during　severe　pollution　episodes　in　fall　and　winter　2016-2017　using　routine　observations　and　the　TrajStat　model.　Results　showed　that　13　heavy　pollution　events,　each　lasting　at　least　two　days,　occurred　in　Beijing.　Of　these,　approximately　61.5%　occurred　in　winter,　characterized　by　heavier　pollution　concentrations　and　longer　durations　than　those　occurring　in　autumn.　A　low-pressure　gradient,　high　humidity,　low　surface　wind　speed,　low　boundary　layer,　and　particular　terrain　(i.　e.,　being　surrounded　by　mountains　on　three　sides)　all　contributed　to　the　high　occurrence　frequency　of　severe　pollution　episodes　in　autumn　and　winter.　During　the　pollution　episodes,　the　average　ratio　of　PM2.5　to　PM10　reached　0.86.　The　air-masses　during　the　accumulation　stage　were　mainly　transported　from　the　northwest,　west,　southwest,　and　southeast　of　Beijing.　The　southwestern　and　southeastern　transmission　paths　accounted　for　21.6%　of　the　total　pollution　load.　In　addition,　the　WRF-CAMx　model　was　used　to　quantitatively　analyze　the　contributions　of　local　and　external　sources　to　the　concentrations　of　PM2.5　in　Beijing　during　16-22　December　2016.　Based　on　this　analysis,　PM2.5　contributions　notably　varied　with　different　air-masses;　in　the　case　of　southern　air-masses,　external　sources　dominated　the　PM2.5　concentrations　in　Beijing　and　local　contributions　decreased　rapidly;　in　contrast,　in　the　case　of　northwestern　air-masses,　the　opposite　pattern　occurred.　Overall,　the　contribution　of　local　sources　to　PM2.5　concentrations　in　Beijing　varied　from　16.5%　to　69.3%　during　the　monitored　pollution　episodes.　©　2019,　Science　Press.　All　right　reserved.